1. Field of the Invention
The invention relates generally to communication units in a digital communication network, and more particularly, to a communication unit that communicates via Integrated Services Digital Network (ISDN) line. The invention has particular applicability to a caller identification unit in the responding communication unit and the identification method.
2. Description of the Background Art
A telephone set connected to a subscriber line is one of the conventional communication units. Regarding telephone sets, there is an automatic answering telephone set that can record the caller's speech at the time of the responder's absence.
This automatic answering telephone set is capable of sending a given message to the caller and record the caller's message automatically, in response to a reference at the time of the user's absence. The user can reproduce the messages of the callers who called during the user's absence by operating a reproduction switch.
Unfortunately, this requires communication charge unnecessarily because the line of the automatic answering telephone set is in a connected state at the time.
Also, the user could not find out who the caller was if the caller did not record a message.
Furthermore, even though the caller's message was reproduced, there were times when the user could not make out where he/she should correspond, just from the message despite the identification of the caller.
There was also an inconvenience that every caller could not be identified unless all the messages of all the callers were reproduced, which would waste some period of time.
A communication unit connected to the ISDN line comprising a memory for storing the calling codes of the caller during the absence of the user is disclosed in Japanese Patent Laying Open No. 64-89641. This communication unit of the prior art comprises a unit for setting the absence state of the user, a memory device for storing the caller's telephone number transmitted via the ISDN line in response to the set up absence state, and a unit for reading the telephone number from the memory device and sending the same as the responding side specification information in the calling information to the ISDN line in response to a line connection instruction applied by the user.
This prior unit requires the set up of the user's absence state. If this absence state is not set up, the calling code of the caller could not be stored at the time of the absence of the user.